The present invention relates to the hot forming of metals, and more particularly relates to the continuous casting and hot forming of the as-cast bars of certain impure or alloyed metal prone to crack during hot-rolling.
It is well known that metals, such as aluminum and aluminum alloys, may be continuously cast, either in stationary vertical molds or in a rotating casting wheel, to obtain a cast bar which is then immediately hot formed, while in a substantially as-cast condition, by passing the cast bar exiting the mold to and through the roll stands of a rolling mill while the cast bar is still at a hot-forming temperature. It is also well known that the as-cast structure of the metal bar is such that cracking of the cast bar during hot forming may be a problem if the cast bar is required to be directly hot formed into a semi-finished product, such as redraw rod, during which the initially large cross-sectional area of the cast bar is substantially reduced by a plurality of deformations along different axes to provide a much smaller cross-sectional area in the product.
While this problem could be avoided by casting a cast bar having an initially small cross-sectional area which need not be substantially reduced to provide the desired cross-sectional area of the final product, this approach is not commercially practical since high casting outputs, and therefore low costs, can be readily achieved only with cast bars having large cross-sectional areas which are rapidly reduced to the smaller cross-sectional areas of the products, such as 3/8" diameter rod for drawing into wire, by a minimum number of severe deformations. Thus, the problem of a cast bar cracking during hot forming must be solved within the commercial context of cast bars having initially large cross-sectional areas which are then hot formed into products having small cross-sectional areas by a series of reductions which often are substantial enough to cause cracking of the cast bar under certain conditions.
For example, U.S. Pat. No. 3,317,994, and U.S. Pat. No. 3,672,430 disclose that this cracking problem can be overcome in copper by conditioning relatively pure copper cast bar by initial large reductions of the cross-sectional area in the initial roll stands sufficient to substantially destroy the as-cast structure of the cast bar. The additional reductions along different axes of deformation, which would cause cracking of the cast bar but for the initial destruction of the as-cast structure of the cast bar, may then safely be performed. This conditioning of the cast bar not only prevents cracking of the cast bar during hot forming but also has the advantage of accomplishing a large reduction in the cross-sectional area of the cast bar while its hot-forming temperature is such as to minimize the power required for the reduction.
The prior art has not, however, provided a solution to the cracking problem described above for metals, such as aluminum, containing a relatively high percentage of alloying elements. This is because the large amounts of alloying element in the grain boundaries of the as-cast structure cause the cast bar to crack when an attempt is made to substantially destroy the as-cast structure with the same large initial reduction of the cross-sectional area of the cast bar that is known to be effective with relatively pure metal. Moreover, the greater the percentage of alloying elements in the cast bar, the more likely it is that cracks will occur during hot forming.